Propped Fracturing in Deep Naturally-Fractured Tight Carbonate Reservoirs

Abstract

Abstract Over the last ten years, thirty wells were performed by proppant fracturing in deep naturally fractured tight carbonate reservoir in Tarim oilfield, including twenty-two hydraulic fracturing wells and eight acid-carrying-proppant fracturing wells. This paper will introduce what we have done for the proppant fracturing, what affect the fracturing and post-production, and why acid-carrying-proppant fracturing was tested and its result. Natural fractures and vugs are the main hydrocarbon reservoirs in Tarim oilfield, and acid fracturing was used to increase the production. But post acid-fracturing production was low in the tight carbonate with fissures, so proppant fracturing was referred to the poor formation with the purpose of increasing prop- length, enlarging discharge area and maintaining post rate in vertical wells. In order to realize the successful hydraulic fracturing in deep openhole wells, some measurements were taken to ensure the treatments such as large scale pad-fluid, 2-6 stages slugs, high pumping rate under the safe treating pressure, small-sized high-strength ceramic proppants and long step concentration with low-medium proppant. There were eight wells sand screenout or sand screenout behavior during treatment in the twenty-two hydraulic fracturing wells, which is mainly attributed to the presence of natural fractures. The hydraulic fracturing's difficulty was depended on natural fractures, there were seven fracturing wells with annulus pressure no packer, four wells' casing pressure increased 10 MPa-25 MPa because of the natural fractures and three wells' casing pressure increased only 4 MPa-6 MPa for no or poor natural fractures while fracturing. Micro seismic results showed that multi-fractures propagated while fracturing, the extending direction of secondary fractures had a large angle with the principal fracture, which ultimately leads to complex propped fractures. The post-fracturing temperature logging shows that the fracture height is about 30 m. The post-fracturing evaluation demonstrates that the half-length of the artificial fracture is 100 m-150 m. Meanwhile, seven acid-carrying-proppant fracturing with the purpose of acid etching fracture and propping fracture were treated in the fissure-matrix carbonate, and the results were not all good for the deep vertical carbonate wells. Natural fractures and holes are important for post-rate of proppant fracturing, some well even has been producing for 10 years, for reservoirs with rare natural fractures and dissolved pores and caves, the post-fracturing production is low or declines rapidly.